1. Field of the Invention
This invention relates to an electrical circuit for driving a piezoelectric transducer which drives an ultrasonic wave generating device employing a piezoelectric transducer as an electro-mechanical conversion element at a frequency equal to or near the natural resonance frequency thereof.
2. Description of the Prior Art
As ultrasonic wave generating devices have been extensively employed in various industrial fields recently, the development and study of the materials of electro-mechanical conversion elements and their manufacturing methods have been advanced, as a result of which electro-mechanical conversion elements having higher efficiency and capable of withstanding greater amplitudes than conventional ones are available. Accordingly, the ultrasonic wave generating device which had been bulky in view of its strength has been improved to be compact, and along with this improvement the oscillator for driving the ultrasonic wave generating device has been improved.
In the ultrasonic wave generating device, especially the piezoelectric transducer has been significantly improved so as to be compact. However, in the case where such a compact piezoelectric transducer is operated by ultrasonic energy, for instance in the case where the piezoelectric transducer is employed in a liquid atomizing device, the following difficulty is involved;
An ordinary piezoelectric transducer can be represented by an electrical equivalent circuits as shown in FIG. 1, which comprises a capacitance Cd as a capacitor independent of vibration, an inductance Lm and a capacitance Cm which are provided by the vibration of the piezoelectric transducer, and a load R in which energy is consumed as the loss in the transducer and the actual mechanical vibration output thereof. The natural resonance frequency of the electrical equivalent circuit is a frequency at which the absolute value of reactance in the inductance Lm is equal to the absolute value of reactance in the capacitance Cm. The transducer has the characteristic that when mechanical load is applied to the transducer vibrating at a frequency near the resonance frequency, the resistance component of the electrical equivalent circuit is varied, the resistance component R increasing with the load. If the piezoelectric transducer with this characteristic is driven with a constant voltage having a frequency equal to or near the resonance frequency, then electrical power inputted to the piezoelectric transducer is decreased with increasing load; that is, as the load is increased, the mechanical vibration output is decreased. This characteristic is disadvantageous in the case where it is required to maintain the mechanical vibration amplitude constant irrespective of the load variation. For instance, the following drawback can be pointed out: If, in an ultrasonic atomizing device employing the piezoelectric transducer as its ultrasonic atomizing vibrator, the piezoelectric transducer is driven with a constant voltage and the supply of a liquid to be atomized at the mechanical vibration output end, or the vibration surface thereof, is gradually increased, then the resistance component R of the piezoelectric transducer is increased, and therefore the electric power inputted to the piezoelectric transducer is decreased, as a result of which the mechanical vibration amplitude is reduced, and accordingly the capability is lowered, and at worst the atomization is not effected.
This non-atomization phenomenon is due to the formation of a thick liquid film on the atomizing surface of the vibrator by the interfacial tension of the liquid and the vibrator. In this case, the load as viewed from the vibrator is considerably great. In addition, the resistance component R as viewed from the terminal of the piezoelectric transducer is also considerably high. Even if the supply of the liquid is suspended, the thick liquid film is held as it is. An electrical input sufficient to atomize the liquid thus held is not applied to the piezoelectric transducer, and therefore it is considerably difficult to atomize the liquid again. In the case where the supply of liquid is decreased, the resistance component R is decreased, and therefore the greater electric power is applied to the piezoelectric transducer. As a result, the amplitude of the mechanical vibration of the piezoelectric transducer becomes great to the extent that it is unnecessary for atomization of the liquid. In the extremely worst case, cavitation is observed in the liquid supplied, thus splashing the supplied liquid directly and increasing the diameters of the atomized particles. Thus, the atomization is not carried out suitably. This is another drawback. Furthermore, since the transducer of the ultrasonic atomizing device is driven at its natural resonance frequency, the current supplied to the transducer is increased as the load is abruptly decreased and therefore the transducer is driven with an abnormally great amplitude. Accordingly, sometimes the transducer is broken. However, it is not practical to change the dimensions of the transducer to increase its strength, because it is necessary to vibrate the transducer at its natural resonance frequency and the resonance condition is disestablished if the dimensions are changed. Accordingly, in order to increase the strength to eliminate the above-described difficulty, it is necessary to selectively use materials in forming the transducer in view of the strength thereof. That is, it is required to use a material high in strength, and the degree of freedom in selecting the material is limited. This is another drawback.
In order to eliminate the above-described drawbacks, for the conventional device, a method is employed in which, as shown in FIG. 2, AC current driving the piezoelectric transducer is detected so that the driving AC current is maintained constant at all times irrespective of the load variation. More specifically, the conventional device comprises a DC electric source 1, an electric source control circuit 2, a voltage and power amplifier circuit 3 (hereinafter referred to merely as "a power amplifier circuit" 3), a piezoelectric transducer 4, a current detecting circuit 5, a DC conversion circuit 6, a voltage comparison circuit 7, and a reference voltage generating circuit 8.
In this conventional device, upon application of a suitable voltage from the DC electric source 1 through the electric source control circuit 2 to the power amplifier circuit 3, the output of the power amplifier circuit 3 drives the piezoelectric transducer 4. The AC current applied to the piezoelectric transducer 4 is detected by the current detecting circuit 5, and the detection signal is applied to the power amplifier circuit in a positive feedback mode. Thus, an oscillation circuit 9 is formed which oscillates at a frequency equal to or near the resonance frequency of the piezoelectric transducer 4. The output of the current detecting circuit 5 is applied to the DC conversion circuit 6, where a DC voltage proportional to the AC current in the piezoelectric transducer is obtained. This voltage is compared with a preset reference voltage outputted by the reference voltage generating circuit 8 in the voltage comparison circuit 7. The output of the comparison circuit 7 is employed to control the electric source control circuit 2 so that the electric source voltage to be applied to the power amplifier circuit 3 is varied to control the output of the power amplifier circuit 3, to permit alternate current corresponding to the preset output voltage provided by the reference voltage generating circuit 8 to flow in the piezoelectric transducer 4, and to drive the piezoelectric transducer 4 with a constant current.
In this connection, it is assumed that the piezoelectric transducer 4 is driven at a frequency equal to or near the natural resonance frequency and a suitable quantity of liquid is supplied to the mechanical output end thereof for atomization; that is, the transducer is operated in steady state. If, under this state, the supply of liquid is increased to increase the load of the piezoelectric trnasducer, then the resistance component R of the equivalent circuit shown in FIG. 1 is increased. However, in this conventional circuit, as the constant current is allowed to flow irrespective of the load variation, the greater electrical energy is supplied to the piezoelectric transducer, and therefore the problems that, when the supply of liquid is changed, the atomization is not effected or the diameters of particles obtained by the atomization are extremely increased can be avoided.
However, this conventional device is still disadvantageous in the following points: When the piezoelectric transducer is broken, or when the lead wires thereof are shorted and the output transistor of the power amplifier 3 is damaged, no current will flow in the piezoelectric transducer 4. Also, when the energy stored in the inductive impedance components of the circuits is discharged by the on-off operation and the transistor in the power amplifier 3 is secondarily damaged to cause a short-circuit trouble, no current will flow in the piezoelectric transducer 4. In these cases, the output of the DC conversion circuit 6 becomes zero and therefore the output of the voltage comparison circuit 7 acts on the electric source control circuit 2 to increase the output of the latter. However, as the value of the current in the piezoelectric transducer 4 is maintained unchanged, zero, the output current of the electric source control circuit 2 is increased more and more, thus damaging the electric source control circuit 2. In the conventional device shown in FIG. 2, a DC electric source and control section consisting of the DC electric source 1, the electric source control circuit 2, the voltage comparison circuit 7 and the reference voltage generating circuit 8, and a high frequency section consisting of the oscillation circuit 9 and the DC conversion unit 6 form a feedback loop, and are in close association with each other. Accordingly, it is difficult to make the design, adjustment and experiment of the conventional device with the two sections separated from each other.
Aside from the example shown in FIG. 2 the same stable atomization can be effected by providing an AC constant current circuit in the power amplifier circuit or between the power amplifier circuit and the piezoelectric transducer. However, this method is disadvantageous in that the control is effected after the direct current has been converted into a high frequency current, thus causing loss of electric power when compared with the case where the control section is provided in the DC electric source section, and therefore it is necessary to increase the output of the oscilllation circuit to a relatively high value, which leads to the use of expensive components and to an increase in collective electric power loss.